LOG114EVM Datasheet, PDF (10/21 Pages) Texas Instruments – LOG114EVM User Guide

Connecting to the LOG114EVM
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In the case where I₁ = 500 nA = I_REF, the V_LOGOUT terminal is equal to the level shifting bias voltage of 2 V
and the V_OUT terminal is equal to 1.5 V. The circuit in Figure 10, while convenient for creating an input
current, does not take advantage of the high-current linearity correction circuitry. To use this circuitry, a
second power supply must be used and configured as shown in Figure 11.
+ VIN
Figure 11. Using a Resistor and the High Current Linearity Correction Circuitry
Changing the input voltage or the resistor value changes the input current I₁ (I₁ = V_IN/R) and results in a
change for the V_LOGOUT terminal voltage according to Equation 5.
( ^VLOGOUT ^{= 0.375·LOG}10
I₂
I₁
+ 2 V + I₁·50 W
(5)
For input currents less than 1 mA, this equation simplifies to Equation 2.
The voltage at the V_LOG terminal is given by Equation 6:
( ^VLOG ^{= [0.375·LOG}10
I₂
I₁
+ I₁·50 W] · 0.3 V/V + 2 V
(6)
The voltage at the V_ERR terminal is given by Equation 7:
V_ERR = 2 +
(I₁·3)
1 kW
+
2
21.5
kW
· 5 kW
(7)
The voltage at the V_OUT terminal is given by Equation 8:
V_OUT = 2·V_LOG - V_ERR
(8)
10
LOG114EVM User Guide
Copyright © 2011, Texas Instruments Incorporated
SBOU110 – September 2011
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